Yarns to be used as pile in cut pile carpets are conventionally ply twisted from two or more component carpet yarns in a true twisting operation which operates at a relatively low linear speed of about 40 to 70 yards (37 to 64 meters) per minute. The plied yarns are then heated by steam while relaxed on a moving belt in an enclosure for sufficient time to heatset the yarns in the ply twisted configuration so that when they are tufted into carpet and cut, the plies will remain in their twisted condition without separating and matting during wear.
Various ways of preparing yarns for cut pile have been proposed to produce yarns at higher speeds yet obtain adequate tuft integrity. The process of Norton and Windley U.S. Pat. No. 3,968,638 entangles the yarn or yarns highly in a hot fluid entangling jet then heatsets the yarn in a twisted condition by a fluid false twisting jet and heater so that although the yarn has substantially no twist while it is wound on a package, the yarn will return to its twisted configuration when the pile yarn is cut and heated during dyeing, thus producing coherent twisted tufts in the final carpet.
A yarn which is plied by false twisting each of two or more yarns, joining them together, and permitting the yarns to ply twist together is described in Chambley and Norris U.S. Pat. No. 4,074,511 and related patents. Coherent yarn may also be prepared by false twisting and heat adhering a thermoplastic carpet yarn in accordance with Tajiri et al U.S. Pat. Nos. 4,355,592 and 4,452,160. All of the above references have the object of producing cut pile tufts having tips of substantially equal coherency so that the carpet has a uniform surface appearance of coherent tufts.
A particular type of carpet known as "frieze" is made by ply-twisting two or more yarns with a higher degree of ply twist than singles twist. When such yarn is heat set in skein form, the unbalanced twist causes the yarn to form kinks known as "twist pigtails" and to be heat set in this configuration. After such yarn is tufted into cut pile carpet, the plies of tufts which are cut at a pigtail location separate at least slightly and at least one ply forms a curl in the plane of the carpet surface, while tufts which are cut elsewhere form coherent tuft tips. This varied surface appearance is desired for some styles of carpets, but the need for higher twist makes the yarn process even slower and more expensive than usual twist plying.